Al-Qaida elements fighting with rebels in Syria constitute the most serious terrorist threat to Britain, and if they were to get their hands on Syria's chemical weapons the consequences could be catastrophic, according to British spymasters.

Russia said on Wednesday it had proof Syrian rebels used the nerve agent sarin in a missile attack on a government-controlled suburb of Aleppo in March. The British prime minister, David Cameron, said last month that al-Qaida-linked elements in the rebel movement had tried to capture chemical weapons for probable use in Syria.

Britain's security and intelligence chiefs "assess that al-Qaida elements and individual jihadists in Syria currently represent the most worrying emerging terrorist threat to the UK and the west," says the ISC.

It adds: "There is a risk of extremist elements in Syria taking advantage of the permissive environment to develop external attack plans, including against western targets. Large numbers of radicalised individuals have been attracted to the country, including significant numbers from the UK and Europe."

They are likely to acquire "expertise and experience which could significantly increase the threat posed when they return home," says the ISC report. Intelligence officials believe about 100 British Muslims have gone to Syria to join rebel groups.

The ISC reports serious concern about the security of what it calls the "vast stockpiles" of chemical weapons held by the Assad regime.

They are believed to include sarin, a clear liquid which attacks the nervous system, ricin, mustard gas and VX – described as "the deadliest nerve agent ever created".

The MI6 chief, Sir John Sawers, told the committee there was the risk of "a highly worrying proliferation around the time of the regime fall".

The committee adds: "There has to be a significant risk that some of the country's chemical weapons stockpile could fall into the hands of those with links to terrorism, in Syria or elsewhere in the region. If this happens, the consequences could be catastrophic."

The shape of the terrorist threat is "potentially changing from tightly organised cells under the control of structured hierarchies to looser networks of small groups and individuals who operate more independently," says the ISC.

It points to a growing threat of attacks by "lone actors", such as the stabbing of Labour MP Stephen Timms while holding a surgery in his east London constituency in 2010.

Lone actors are much harder to detect – something al-Qaida appears determined to exploit. The ISC refers to evidence from an unidentified Home Office official who told the committee: "There is no doubt that the more sophisticated people in al-Qaida recognise that groups are, in some ways, a thing of the past; and that encouraging lone acts of terror is exactly the way forward."

In a passage studded with asterisks marking redactions from the full report, it says the foreign secretary, William Hague, told the ISC: "We don't believe that while we are engaged in this process of sanctions and negotiations and a twin-track policy it would be right to launch a military strike on Iran and we've said that very clearly to the Israelis."

It refers, intriguingly, to how British spies have "become more creative in how they maintain and develop accesses to supply the government's intelligence requirements" targeting Iran.